1. Field of the Invention
The present invention relates to a steering assist apparatus for a vehicle, and more particularly, to a steering assist apparatus configured to adjust a force relating to steering, to thereby assist the steering.
2. Description of the Related Art
In a vehicle such as a motor vehicle, a steering assist apparatus configured to adjust a force relating to steering, such as a steering assist force, to thereby assist the steering is generally installed. The steering assist apparatus of this type is desirably configured to not only simply reduce a steering load on a driver, but also adjust the force relating to the steering so as to reflect an intention of the driver.
For example, in International Patent WO2014/087546A filed by the applicant of the present invention, there is described a steering assist apparatus configured to calculate a steering power, and changing the adjustment of the force relating to the steering depending on whether or not a magnitude of the steering power is larger than a reference. In particular, the steering power is calculated as a sum of a product of a steering angular velocity and a steering torque and a product of a steering angle and a derivative of the steering torque. The steering assist apparatus described in International Patent WO2014/087546A can adjust the force relating to the steering so as to suitably reflect the intention of the driver compared with a case in which the magnitude of the steering power is not considered.
However, even when the determination is made as to whether or not the magnitude of the steering power is larger than the reference based on the configuration described in International Patent WO2014/087546A, in some cases, the adjustment of the force relating to the steering cannot be appropriately changed depending on a steering state.
For example, even when the steering is started under a state in which the steering wheel is in a neutral area (a straight traveling position of the vehicle and a neighborhood thereof), the steering torque is not generated immediately, and the magnitude of the steering power is very small in a steering area in which a steering extent with respect to the neutral position is small. Therefore, even when the determination is made as to whether or not the magnitude of the steering power is larger than the reference under the state in which the steering wheel is in the neutral area, the power relating to the steering cannot be appropriately changed depending on the steering state.
Moreover, when the driver rotates the steering wheel in a neutral area while resisting a self-aligning torque, an appropriate resisting force or attenuation force is preferably applied to the rotation of the steering wheel so that the driver can stably carry out the steering operation. In contrast, when the steering wheel is rotated in a neighborhood area of the neutral position toward the neutral position by the self-aligning torque, the resisting force against the rotation of the steering wheel is preferably small so that the steering wheel quickly returns to the neutral position.
However, when the steering wheel is rotated in a neighborhood area of the neutral position toward the neutral position, the configuration described in International Patent WO2014/087546A cannot determine whether the rotation of the steering wheel is caused by the operation of the driver or the self-aligning torque. Thus, the resisting force against the steering and the like cannot be appropriately changed depending on the steering state. Moreover, in order to distinguish the above-mentioned states from each other, it is necessary to determine whether or not a state in which the driver has released the steering wheel, namely, a hands-free state exists.
Further, when the steering wheel is rotated by the steering operation of the driver as well as the force received by the steered wheels from a road surface, such as the self-aligning torque, and the rotation directions thereof are the same, the steering torque results in a low value. Therefore, the magnitude of the steering power is very small, and hence even when the determination is made as to whether or not the magnitude of the steering power is larger than the reference, the adjustment of the force relating to the steering cannot be appropriately changed depending on the steering state.